The present invention relates to the quantitative measurement of gas entrapped in drilling fluids during the drilling operation in an oil well. More particularly, this invention relates to improved methods and apparatus for performing quantitative measurements of such entrapped drilling mud gas using gas trap sampling systems such as that of U.S. Pat. No. 5,199,509 which is assigned to the assignee of the present invention.
Drilling mud is pumped down the drill string in a rotary drilling operation. It serves multiple purposes. It cools and lubricates the drill bit at the end of the drill string. It balances pressure between the earth formations and the borehole. It carries "cuttings" of drilled rock to the surface and it collects and entraps fluid samples from penetrated earth formations of oil, gas and water. Drilling muds are typically comprised of solids entrained in a water based or oil based slurry of weighting fluids such as barite or the like.
Mud logging is the term which has been applied to the surface monitoring of the entrapped fluids and gasses in the drilling mud when it is circulated to the surface during a drilling operation. In order to extract and measure light hydrocarbon gasses from the return flow of drilling mud, samples of the mud are collected in a gas trap.
The-purpose of the gas trap is to quantitatively measure the amount of gas in the drilling fluid. This measurement has presented a problem with prior art gas traps because of air and gas leaks through the fluid exhaust port, which is generally open to the air outside the trap, and leakage around the stirrer bar motor shaft, the stirrer being used to agitate the drilling fluid inside the gas trap so that it will release entrapped formation gasses.
The aforementioned U.S. Pat. No. 5,199,509 describes an improved gas trap system which overcomes some of the measurement problems by provision of a discrete air vent line whose far end is in gas-free air and by reducing or eliminating uncontrolled gas phase mixing at the mud exit port and from the agitator shaft seal leaks. However even this system is improved upon further by use of the methods and apparatus of the present invention.
In order for a gas measurement system to be quantitative, gas separated from the drilling fluid by the gas trap must be drawn through a sample line at a known flow rate. The gas is then measured using a gas chromatograph and a total hydrocarbon analyzer. Gas-in-air measurements of the gas from the gas trap can then be related back to actual gas-in-mud levels. Periodic gas-in-mud measurements must be made in order to "calibrate" a quantitative gas trap. This calibration can consist of collecting a known volume of drilling fluid at the gas trap and then distilling that sample in a Steam Still such as that of U.S. Pat. No. 3,050,449 which is incorporated herein by reference. The distilled gas is then measured and compared with gas measurements direct from the trap to determine what the relative recovery is from the gas trap for each gas component of interest.
In order for a gas measurement system to remain stable enough to relate gas-in-air to gas-in-mud certain parameters of the gas trap need to be monitored. These are:
1) The flow rate of the sample being pulled from the gas trap should be known and remain constant; PA0 2) the speed of the gas trap agitator should be known and remain constant; PA0 3) The gas trap must be sealed against wind effect to prevent uncontrolled sample dilution; PA0 4) Gas evolved from the drilling fluid should not exceed the sample rate which would cause gas trap saturation; and PA0 5) The gas trap should not be sensitive to mud level changes caused by gas trap immersion variations.
Sealed gas traps can saturate at high gas-in-mud levels when total gas evolution rate exceeds the trap sample flow rate. The agitation action of the gas trap can create bubbles in the drilling fluid entering the trap. Such bubbles can recapture and carry off some of the gas evolved from the sample. Traps equipped with air powered motors can develop leaks in the bearings of the motor shaft allowing air to enter directly from the motor shaft into the sealed gas trap. This can dilute the gas sample at unknown rate.